Traumatic brain injury (TBI) can result in the disturbance of cognitive, behavioral, emotional, and physical functioning. The induction of both synaptic plasticity and memory is thought to depend on the balance between opposing molecular regulatory factors, including protein kinases and phosphatases. While there has been considerable investigation on the roles of kinases, there is a significant gap in knowledge regarding the role of protein phosphatases in TBI pathology. Calcineurin (CN) is a calcium/calmodulin-dependent Ser/Thr protein phosphatase that is enriched in neural tissue. While CN is known to have many cellular functions, this project will focus on its role in two well-documented mechanisms of TBI: Bcl-2 associated cell death and long-term plasticity. Recent studies demonstrate that inhibition of CN facilitates memory performance in normal animals. Therefore, we posit that late inhibition of CN may attenuate memory deficits after TBI. This project will also determine whether CN inhibition can attenuate activation of the mitochondrial cell death pathway that has been documented to occur both early and late after TBI in standard brain regions. Acute and chronic inhibition of CN is hypothesized to attenuate apoptotic hippocampal neuronal death by the dephosphorylation of key cytosolic components, such as the Bel family member Bcl-2- associated death protein (BAD). Chronic inhibition of CN is hypothesized to enhance recovery of cognitive deficits by reducing CN's inhibitory effect on protein kinase A (PKA)-induced phosphorylation of the transcription factors CREB and NFAT and subsequent expression of proteins involved in synaptic plasticity. In addition to evaluating the therapeutic value of CN inhibition, this project will determine the effects of TBI on CN and its substrates that are associated with regulators of neuronal survival and plasticity after controlled cortical impact (CCI) in rats. We will also investigate whether CN's role in the BAD dephosphorylation induced caspase-3 activation after CCI. To further establish whether CN acts as an inhibitory constraint on postinjury memory, we will examine the effects of genetically inhibiting CN using a doxycycline-dependent rtTA system to express a CN inhibitor reversibly in the mouse brain. The reversible aspect of this mutant model will allow us to determine whether any beneficial effects of CN inhibition are sustainable. Lastly, we will examine the relationship between CN levels in human TBI tissue samples and injury severity and outcome.